Polymer films are used in many fields for numerous different uses. Countless different properties are required or desirable depending on the applications in which the films are used. One of the many aesthetic and functional characteristics of a film is the presence and nature of text, images, indicia and other aspects of printed appearance. These function to provide identifying or other useful information or to give a particular appearance for visual appeal or other reasons. They are of particular use where the film is used in packaging.
Many different techniques can be used for printing or marking films and related substrates. Traditionally, substrates have been marked by applying inks and using various printing techniques. More recently, films have been marked using alternative techniques. One such alternative technique is laser marking, whereby laser-sensitive components have been applied to or incorporated within substrates such that laser irradiation can bring about a change in appearance by causing a change in the laser-sensitive components. Laser marking or writing can bring advantages in terms of cost and performance. The desired mark or image can be “printed” without ink, merely by “writing” with a laser. Laser writing is precise and quick, and can be used with materials which are not necessarily flat or uniform. The surfaces of the substrate in most cases do not have their physical characteristics adversely altered (because they do not come into contact with conventional printing apparatus) and the laser writing techniques may be used even where the laser-sensitive components are embedded within the substrate.
One example of a laser-sensitive coating composition is disclosed in WO 2009/024497. This document discloses a composition comprising titanium dioxide in the anatase form and a polymeric binder. The document refers to ultra-violet, visible or infra-red laser irradiation, preferably infra-red laser irradiation. IR absorbers may also be present, for example tungsten suboxide, tungsten bronze, or mixtures of tungsten trioxide, tungsten bronze and metallic tungsten.
Amongst chemicals which can change appearance upon irradiation are certain diacetylene-containing compounds, as disclosed in WO 2009/093028 for example. This document discloses diacetylenes which are poly chromic, i.e. which change colour upon irradiation. The polychromic diacetylenes may be present in or on a material so that colour may be imparted to the material, or the colour of the material may be changed upon irradiation. The document discloses that preferred compounds are those which are initially colourless or of low visual colour and which become coloured upon irradiation, and/or compounds which undergo multiple colour changes. For example, the compounds may change from being initially colourless or of low visual colour to become coloured upon irradiation and subsequently to change to a different colour upon further irradiation with the same or different type of radiation. Types of radiation include laser or non-coherent, broadband or monochromatic radiation, ultra-violet, near, mid or far infra-red, visible, microwave, gamma-ray, x-ray or electron beam radiation. The document discloses that, in addition to the diacetylene compounds, there may also be incorporated other compounds which undergo colour change reactions on irradiation, for example “leuco dyes”. The document discloses that the poly chromic substances may be included in a surface coating formulation or within the bulk of the substrate. The substrates can include thermoplastics.
WO 2009/081385 also discloses diacetylene-containing poly chromic materials, and in particular thermoplastic materials comprising polymers and these polychromic substances. The polymers may be polyolefins such as polyethylene, polyethylene terephthalate, polypropylene, or mixtures thereof.
WO 2012/114121 relates to the reversible activation of certain diacetylenes. These undergo a topochemical polymerisation reaction to give coloured diacetylenes only when they are simultaneously exposed to additional activating stimuli. This document discloses that reversible activation is advantageous because the compounds have high environmental stability in coatings or in plastics parts. The diacetylene compounds are applied to or incorporated within substrates, the substrates are then exposed to a first activating stimulus which converts the diacetylene compounds from an unreactive form to a reactive form, and subsequently a second stimulus that causes the reactive form of the diacetylene compound to polymerise and form coloured substrates. On removal of the activating stimulus the diacetylene compound reverts to its unreactive form. The substrate may be packaging.
A different type of laser marking is disclosed in WO 2007/141522. In this document, a non-stoichiometric compound such as reduced indium tin oxide (r-ITO) functions as a highly effective absorber of near infra-red radiation and is useful in combination with a marking component such as an ammonium octamolybdate based ink formulation. The result is to produce a colour-forming reaction in respect of a component that would otherwise undergo the desired reaction on irradiation at a different wavelength.
WO 2010/001171 discloses several different types of diacetylene-containing compounds and their uses to impart colour to materials by subjecting the materials to irradiation. Amongst the preferred colour-forming diacetylenes mentioned in the document are those which are capable of forming at least two distinct colours selected from blue, red, green, cyan, magenta and yellow (particularly preferably those which change from colourless to blue), and those which give rise to electrical conductivity as well as colour on polymerisation.
WO 2007/045912 is particularly concerned with laser imaging of substrates such as paper, card or board. This document discloses a method of marking a substrate comprising the steps of coating the substrate with a white or colourless solution of a soluable alkali or alkaline earth metal salt of a weak acid, and irradiating areas of the substrate to be marked such that those areas change colour.
WO 20006/114594 discloses an example of a printing system and apparatus for the laser marking of a substrate. The apparatus comprises a laser diode for emitting a beam of laser light and a galvanometer for aligning a desired point on the substrate with the laser beam such that the laser beam irradiates the desired point thus causing an additive to change colour at said point. The document discloses that the system may be used on a wide variety of substrate materials, for example, metals, alloys, glasses, ceramics, plastics, fabrics, wood, paper, card, resins, rubbers, foams, composites, stone and edibles.
WO 2006/114600 similarly discloses a substrate marking system and apparatus. The emphasis on this document is on multi-colour printing. Additives are used which are susceptible to changing colour to one of at least two selectable colours upon irradiation, each selectable colour being different from the colour of the additive prior to irradiation. As well comprising a laser diode for emitting a beam of laser light and means for aligning a desired point on the substrate with the laser beam, the apparatus also comprises a means for controlling a fluence level of the laser beam to select the resultant colour of the additive from the selectable colours.
WO 92/07297 discloses a laser imagable composition comprising a particular combination of certain polyacetylenic compounds and certain polycarbocyanine dyes. The compositions are prepared under atmospheric conditions by forming a dispersion, emulsion or suspension, preferably an aqueous dispersion in a binder to provide a dispersion containing from about 1 to about 50%, preferably from about 4 to about 20% of solid polyacetylenic microcrystals. The document states that the polycarbocyanine dyes are effective heat transmitting agents and that only small amounts of these dyes are needed to provide desired absorptions. They are said to transmit heat in excess of a critical temperature of the thermochromic polyacetylenes. The polycarbocyanine dyes disclosed in this document are disadvantageous in terms of their optical properties including their colour and transparency. This document does not disclose inorganic energy absorbers/heat transmitters.
WO 2006/051309 discloses a photothermal recording medium which is a colourless or transparent composition comprising a charge-delocalisation compound and a photoacid, wherein the photoacid generates an acid on irradiation or heating, thereby forming a coloured charge-transfer complex with said compound.
WO 2006/113778 is another document which discloses laser activated thermochromic compositions. The document in particular relates to thin films and coatings of such compositions that undergo an irreversible colour change when heated by laser energy. The use of a stabiliser in the form of a radical trap is essential in accordance with this document. Various different types of thermochromic dyes and stabilisers are disclosed.
EP 0 600 441 discloses a laser marking method comprising irradiating laser light on a thermosensitive colour forming ink layer formed on a substrate, the ink layer being formed by printing with a printing ink comprises a leuco dye as a colour former and an acid substance as a colour developer, in which the printing ink further comprises at least one background colour formation inhibitor selected from the group consisting of a water-soluble amino acid, an ammonium salt of an inorganic acid, a pH buffer, and water.
WO 2007/114829 is a further document which is concerned with laser marking and in particular relates to a coating composition which can be used in the product and package labeling field. This document discloses a coating composition comprising electron donor dye precursor particulars encapsulated with a polymer having a glass transition temperature Tg, of from about 150® C. to about 190® C. The document discloses specific electron donor dye precursors as being suitable including fluorine and phthalide compounds. The high temperatures disclosed in this document are disadvantageous in terms of suitability with various polymeric films some of which can only be handled at low temperatures.
WO 2007/057367 discloses tetrabenzodiazadiketoperylene pigments for laser marking. This produces a florescent marking readily apparent under UV light but not readily apparent under ambient light. The document states that this could be useful in for example security marking and brand identification of printed packaging.
EP 1 852 270 relates to laser marking in the context of laminates. One of the layers in the laminate is a transparent thermoplastic resin exhibiting good light transmittance. Another layer comprises a thermoplastic polymer composition containing a chromatic colorant and a black substance in particular ratios. The laminate can be marked in two or more different colour tones by irradiating with two or more laser lights having different energies from each other.
EP 0 764 548 discloses a thermosensitive recording adhesive label sheet comprising various components. A layer is included which comprises an electron donating colouring compound (leuco dye) serving as a colouring agent and an electron accepting compound serving as a colour developer (capable of inducing colour formation in said leuco dye upon application of heat thereto). Other components in the product include a support, a protective layer, an adhesive layer and a disposable backing sheet.
WO 2010/112940 is another document which relates to the use of certain diacetylenes in laser imaging. This document discloses applying activable colour forming compounds to substrates wherein the activable colour forming compounds are initially unreactive but become reactive upon activation. The colour forming compound may be activated in the areas of the substrate were the image is to be formed, followed by reacting the activated colour forming compound into its colour form to produce an image.
WO 2007/003030 discloses acetal copolymers which are thermally reactive near-infra-red absorbing copolymers. The focus is on the production of lithographic printing plates for computer-to-plate and digital offset press technologies, photo resist applications, rapid prototyping of printed circuit boards, and chemical sensor development. The copolymer may be used in the preparation of a coating for use in those product.
WO 2006/018640 relates to multi-colour printing using laser marking with polydiacetylene chemicals. The document states that the polydiacetylenes typically exhibit a colour (or a shade of colour) dependent on the degree of polymerisation and therefore that by controlling the degree of polymerisation of a diacetylene, a variety of colours from blue through to red and possibly even yellow can be produced. In other words, multi colour printing can be achieved simply and specifically, especially by using one or more UV lasers. The method in this document comprises applying a combination of a diacetylene and a photo acid or photo base, and polymerising the diacetylene by radiation, preferably UV radiation, in order to form an image. Polymerisation may occur to differing degrees in different areas, and the laser may be tunable.
U.S. Pat. No. 6,376,577 discloses laser-markable plastics which comprise, as dopant, graphite particles having one or more coatings. This enables high contrast to be achieved.
U.S. Pat. No. 5,139,926 and U.S. Pat. No. 5,215,869 disclose preparation of a supported modulating film having a permanent yellow imaged layer of the homopolymer of 10,12-docosadiyndioc acid monomer.
US 2012/00103045 discloses an intrinsically markable laser pigment in the form of a reducible metal compound.
GB-A-2352854 discloses laser markable materials comprising a thermoplastic elastomeric polymer, pigmented with titanium dioxide.
WO 2010/029276 relates to laser imaging and its use in security applications. This discloses a method of forming an invisible indicium on an article that comprises an outer opaque layer and an inner, laser-imagable layer, which comprises irradiating the article with a laser, whereby the laser radiation passes through the opaque layer, and causes the laser-imagable layer to change colour. The article can be scanned as a security check. Thus, laser imaging is said to be useful in order to mark articles with covert indicia in order to prevent counterfeiting, forgeries and ID theft. This is disclosed as potentially being useful with official documents such as passports, identity cards, bank notes, high branded value goods, pharmaceutical compositions, foodstuffs and pin numbers or other access codes.
WO 2005/068207 is a further document relating to the use of functional IR-absorber/colour developers to enhance laser imaging. For example, certain copper salts are useful in order to bring about a colour-forming reaction that would otherwise occur only at a different wavelength.
WO 20/074548 discloses yet further laser-markable compositions wherein the ink composition comprises a solvent, a binder (preferably but not essentially having a labile group) and an oxyanion of a multivalent metal. The metal oxyanion is preferably ammonium octamolybdate.
WO 2006/108745 discloses coating compositions which yield coloured images of good intensity and durability, which can be modulated in order to achieve either transparent or opaque coatings. Various different compounds are disclosed. The document discloses exposing the parts of the coated substrate, where a marking is intended, to energy in order to generate a colour marking. The energy may for example be infra-red irradiation.
WO 2007/088104 discloses a composition which comprises a latent activator and a colour former. The latent activator may be an acid derivative or a salt of an acid and an amine. The colour former may be selected from various groups including phthalides, fluorans, triarylmethanes, benzoxazines, quinazolines, spiropyrans, quinones, thiazines and oxazines and mixtures thereof. The substrate may be marked by coating a substrate with the composition and exposing those parts of the coated substrates, where a marking is intended, to energy (for example UV, IR, visible or microwave irradiation) in order to generate a marking.
WO 2011/089447 is another case relating to apparatus and systems used for inkless printing. There is a substrate which includes material susceptible to change colour upon irradiation, a radiation source operable to produce radiation at two or more distinct wavelengths, and means for controlling the emission of radiation from the radiation source so as to controllably irradiate selected areas of the substrate with desired quantities of radiation from the radiation source so as to mark the substrate in a desired manner.
WO 2008/107345 discloses a laser-sensitive recording material. There is a substrate coated with a recording layer and an undercoating layer. The recording layer undergoes a colour change on heat treatment produced by laser irradiation and the undercoating layer comprises a pigment.
WO 2008/050153 relates to a laser markable composition comprising a marking component and an organic compound, wherein the organic compound absorbs laser light and causes the marking components to change colour. The organic compound is defined in terms of its absorptivity ratio and various possible organic compounds are disclosed.
WO 2007/063339 discloses a laser imagable marking composition comprising a dye responsive to the presence of hydrogen ions but substantially non-responsive to irradiation or heating, a compound that generates an acid on irradiation or heating, and a binder. The acid-generating compound may be responsive to irradiation, for example near infra-red radiation or UV irradiation. The document discloses that effective marking can be achieved in a variety of colours. The composition is stated as being typically initially colourless or transparent and can be used to mark a substrate or polymer matrix effectively, using non-visible radiation.
Yet further examples of diacetylene laser-writable pigments are disclosed in WO 2010/089595 and WO 2011/121265.